Field of the Invention
The present invention relates to a conversion device, particularly to a regulating power converter by sensing the transformer discharge timing.
Description of the Related Art
Due to the fact that internal elements of most electric devices require direct-current (DC) voltages, a power supply is used to convert alternating-current (AC) voltages or DC voltage into the DC voltages such that the electric device operate well. The power supply is divided into nonisolated power supplies and isolated switching power supplies. Isolated power supplier, with an inclusion of a power transformer, is used for most AC to DC, and some DC to DC converters, for safety and other performance considerations. There are various topologies for such converters; examples of such isolated converters topologies including Flyback, Forward, Qusai-Resonant, Full Bridge, Half-Bridge and Push-Pull.
In order to precisely control the output voltage, the isolated converter uses primary side regulation (PSR) or secondary side regulation (SSR) to send information from the secondary side back to the primary side. In SSR, a photo-coupler is often used to serve the purpose. However, the photo-coupler is very expensive. For PSR, the photo-coupler is eliminated. FIG. 1 shows an isolated converter with PSR, and FIG. 2 shows waveforms of voltage and current of the isolated converter with PSR. As shown in FIG. 1 and FIG. 2, the isolated converter comprises a transformer 10, an NMOSFET 12, a resistor 14, a controller 16 and a diode 18. The transformer 10 has a primary winding, a secondary winding and an auxiliary winding. A primary voltage Vp, a secondary voltage Vs and an auxiliary voltage Va are respectively applied across the primary winding, the secondary winding and the auxiliary winding. The primary winding is connected with a voltage terminal having an input voltage Vin and receives a primary current Ip from the voltage terminal. The secondary winding sends out a secondary current Is to a loading capacitor 20 via the diode 18. The NMOSFET 12 receives a gate voltage Vg from the controller 16. An output voltage Vo is applied across the loading capacitor 20.
PSR has a key advantage of providing regulation without secondary side sensing, thus simplifies the power supply by removing secondary output voltage sensing and feedback through the isolation. Instead, the output voltage sensing takes places on the auxiliary winding on the primary side. In traditional PSR design, voltage sensing on the auxiliary winding suffers from its dependency on the accuracy of transformer ratios, since the output voltage Vo is related to PSR sensed voltage Va by the winding ratio of auxiliary winding to the secondary winding. One turn of winding is 360 degrees around the transformer core. During transformer manufacture, a winding ratio error is produced when an actual turn is slightly more or less than a designed one. The winding errors introduced by the first winding and last winding are higher than the middle windings.
The primary winding has a relatively higher number of turns. Since the major winding error only involves the first and last winding, the percent winding error is relatively small. The auxiliary winding has a much smaller number of windings, thus winding error is much more significant. This becomes a critical problem in traditional PSR converters, where the Vo is sensed at the auxiliary winding. The winding ratio of Nerr:1 and Merr:1 is respectively for primary:secondary and auxiliary:secondary. For the next cycle voltage control, the traditional way is to detect the voltage S1/S2/S3 of the auxiliary winding in voltage sampling mode. But the problem of its existing method is the accuracy of the voltage sampling points. If the voltage sampling point is caught too early, such as S1, the secondary measure is too large, and the voltage across the diode 18 will be too large, which causes sampling voltage too large. If the voltage sampling point is caught too late, such as S3, the wrong ring voltage is sampled. Therefore, how to catch the exact point in voltage sampling mode, such as S2 is critical. However, in any case, the error inductance produced Men cannot be changed in voltage sampling mode.
To overcome the abovementioned problems, the present invention provides a regulating power converter by sensing the transformer discharge timing, so as to solve the afore-mentioned problems of the prior art.